1. Field of the Invention
The present invention relates to a technology for producing trichlorosilane (TCS) of high purity, and relates to a method for producing polycrystalline silicon using the method for producing the trichlorosilane.
2. Description of the Related Art
In a process for producing high-purity polycrystalline silicon of a semiconductor grade, generally a “Siemens method” of reducing trichlorosilane (TCS) gas and depositing the product on a silicon rod has been widely used.
As for the Siemens method, National Publication of International Patent Application No. 2004-532786 (Patent Document 1) discloses an invention relating to a method of producing polycrystalline silicon, which newly includes a step of converting disilane (HnCl6-nSi2: n is a value of 0 to 6) that exists in a gaseous effluent coming out from a CVD process for producing a polycrystalline silicon, into mono-silane, in a hydrogenation reaction vessel for converting tetrachlorosilane (STC) into trichlorosilane (TCS). The method is evaluated to have advantages of enhancing a yield of a hydrogenation process by combining a hydrogenation reaction with a thermal decomposition reaction of disilane in the same reaction vessel which is the hydrogenation reaction vessel, and the like. TCS produced in the hydrogenation step is recovered, and can be recycled to a step of depositing the polycrystalline silicon.
Incidentally, a by-product mixture produced when polycrystalline silicon is deposited by reacting TCS with hydrogen also includes a poly-silane (H2(n+1)−mClmSin: n is an integer of 3 or 4 and m is an integer of 0 to 2(n+1)), other than disilane having a silicon atom number n of 2. Accordingly, when these poly-silanes (n=2 to 4) can be effectively used, the by-product mixture produced in the step of depositing the polycrystalline silicon is recycled as a raw material for use in producing the polycrystalline silicon, and more specifically, the by-product mixture is facilitated to be recycled as the TCS, and a yield of the production process is further enhanced.
In other words, a conventional method of producing polycrystalline silicon including a method disclosed in National Publication of International Patent Application No. 2004-532786 (Patent Document 1) has a room to be improved in a process of converting the above described by-product mixture into the TCS as the raw material for use in producing the polycrystalline silicon.
In addition, in order to produce high-purity polycrystalline silicon of a semiconductor grade, it is required to enhance the purity of TCS which is to be the raw material. As a result, it is needed to prepare a step for removing impurities in the TCS and the by-product which are circulated and used in the process for producing the polycrystalline silicon. Accordingly, it is extremely important from a practical standpoint to design the process so as to facilitate the removal of the impurities from the TCS and the by-product which are circulated and used in the process for producing the polycrystalline silicon.